Hitherto, it has been disclosed, for example, in JP-A-55 19023 and JP-A-59 227241, that a lactic fermented soybean milk is prepared by fermentation of a soybean milk with lactic acid bacteria, but that this product has a grassy smell, in other words, a bean smell, which presents a problem for a lactic fermented soybean milk beverage.
On the other hand, it has been disclosed, for example, in JP-A-59 132855, JP-A-62 155076, that for removing the grassy smell and improving the flavor of the fermented soybean milk, a soybean milk obtained by heating soybeans or puverized soybeans to deactivate lipoxygenases contained therein, is either as it is, or with sugars added, subjected to an alcoholic fermentation and a lactic acid fermentation using yeasts and lactic bacteria, to prepare, on the one hand, a lactic fermented acid beverage containing a comparatively small amount of alcohol, and on the other hand, a lactic fermented soybean milk containing a comparatively large amount of alcohol, that is, a fermented soybean milk liquor.
These products are free of the grassy smell and have a good flavor as a result of the synergy between the alcoholic fermentation and the lactic acid fermentation.
Thus, these converted alcohol-containing lactic acid beverages as mentioned above have been improved in smell and taste because of the absence of grassy smell. However, since, not only they contain a foaming carbon dioxide gas, but also, the lactic fermented beverage containing a comparatively small amount of alcohol contains ordinarily about 0.1-3% of alcohol, and the lactic fermented liquor contains 7-8% of alcohol, they bring about inconveniences such as those listed below.
1) They are not beverages suitable for everybody, because they contain alcohol.
2) Since they contain carbon dioxide gas, filling a container with these beverages is difficult. In particular, when some sugar remains in the fermented soybean milk or in the case any additives such as sugars are added to the fermented soybean milk after completion of the fermentation treatment in order to prepare a blended fermented soybean milk product, fermentations caused by live yeasts and lactic acid bacteria take place, so that the amounts of alcohol and carbon dioxide gas are increased, and the qualities of the products are changed. In addition, especially when easily deformable sealed containers, such as packages made of paper or pulp, are used, deformation and expansion of the packed products take place during storage.
3) The amount of carbon dioxide gas dissolved in the fermented soybean milk varies with the temperature, and the amount of the fermented soybean milk in the filled container varies with the temperature, so that packaged products equal in content quality and volume cannot be obtained.
4) When no sugars are present in the fermented soybean milk, or when the remaining sugar has been eaten up by the yeast, autolysis of the live yeast in the fermented soybean milk begins and proteolytic enzymes in the cells of the yeast react with the protein in the soybean milk to product a bitter substance, so that the quality of the product deteriorates at the time of preparation thereof and is further lowered during storage.
Accordingly, a method is desired for preparing a fermented soybean milk which is free from the above-listed inconveniences and has the advantages that it does not contain any alcohol so as to suit everybody's taste, it is always even in quality, and it can be used to fill a container easily to obtain a packaged product which does not contain alcohol, carbon dioxide gas, or a bitter substance, so that it is not subject to expansion and deformation during storage, and thus, is stable and has an excellent shelf life.